Fluorescent lamps are typically composed of a glass tube which contains a small quantity of mercury, a low pressure, inert gas and phosphorous powders which coat the inside part of the tube. At the extremities two electrodes are present which, connected to a suitable driving circuit, create the arc that permits the discharge of the gas to be generated and maintained.
Among the possible driving circuits the so-called high frequency ballast circuits can be enumerated: these are circuits at whose output an alternating voltage signal is generated at a frequency and amplitude necessary to keep the lamp on; this waveform is produced by a circuit that comprises a couple of transistors that switch at a frequency of tens of KHz, a current limiting coil and a filtering capacitance.
When the ageing condition of the lamp approaches, the voltage at the extremities of the same will tend to increase because of the depletion of the emissive coating on the cathodes with the consequent increase in the drop in voltage at their ends. It is common that this phenomenon comes about asymmetrically as one cathode ages before the other; this phenomenon takes the name of “rectifying effect”.
The resulting increase of power dissipated in the lamp could lead to an excessive overheating with dangerous consequences such as the fusion of the glass that surrounds the lamp itself; for this reason the ballast circuits must detect this failure condition, when it exceeds a certain level, and undertake suitable preventive measures such as turning off the ballast.
Various attempts have been made to prevent the overheating of the lamp due to the ageing such as in the EP patent 0 681 414. In said patent a ballast circuit for a discharge lamp 10 is described having two cathodes (FIG. 1) in which the ballast circuit comprises an inverter 1, driven by a device 11, that provides for an alternated voltage at its output terminals; the inverter 1 is fed through a voltage Val coming from a PFC or from a rectification stage 100 fed in turn by the mains voltage Vin. The ballast comprises a circuitry 2 to couple the discharge lamp 10 to said output terminals, another circuitry 3 that measures a direct voltage component Vdc1 at the ends of the blocking capacitor C1 placed between the lamp and ground GND and means 4 suitable for deactivating the inverter 1 when the lamp approaches the ageing conditions. In the means 4 the measured direct component Vdc1 is compared with a signal Vdcm referred to ground GND and produced by means 5; when said component Vdc1 is less or greater than the signal Vdcm by a given value, the means 4 turn off the inverter 1 by acting on the device 11 through a signal Dis.
The proposed solution does not consider a problem linked mainly to the variations of the input voltage of the inverter, whether it be the mains voltage rectified or the output of a stage of the power factor correction (PFC). These variations can be due to low values of the input capacity of the inverter, to short interruptions of the mains voltage that cause a voltage drop or to transitory phenomena that cause its variation. In addition an oscillation at a frequency equal to twice the mains voltage frequency is overlaid to the direct value of the input voltage of the inverter; the amplitude of this oscillation is inversely proportional to the value of the capacity (electrolytic) placed downstream of the rectifier stage (normally a diode bridge) or of the PFC.
The circuitry proposed in the abovementioned patent also intervenes in presence of one of the abovementioned variations of the inverter input voltage, even though the lamp does not present any type of ageing condition.